U.S. patent number 5,267,434 [Application Number 07/868,607] was granted by the patent office on 1993-12-07 for gas turbine topped steam plant.
This patent grant is currently assigned to Siemens Power Corporation. Invention is credited to Dietmar Bergmann, Hermann Brueckner, Jerry Saddler, Heinz Termuehlen.
United States Patent |
5,267,434 |
Termuehlen , et al. |
December 7, 1993 |
**Please see images for:
( Certificate of Correction ) ** |
Gas turbine topped steam plant
Abstract
A gas turbine topped onto two or more steam turbine plants
having therebetween a heat recovery heat exchanger comprising two
side-by-side, but separate, ducts is disclosed. Each of the ducts
comprises heat exchange means for each of the respective steam
turbines. The hot exhaust gases from a gas turbine are passed in
heat exchange with heat exchange means for each of steam turbine.
Damper means for controlling the amount of hot exhaust gas passing
into the respective heat exchange means for each steam turbine are
also disclosed. In another embodiment of the invention, separate
damper means for each of said respective heat exchange means for
varying the amount of hot exhaust gas between each of said heat
exchange means are provided. The inventive system provides high
plant efficiency and excellent operating flexibility.
Inventors: |
Termuehlen; Heinz (Sarasota,
FL), Saddler; Jerry (St. Louis, MO), Brueckner;
Hermann (Uttenreuth, DE), Bergmann; Dietmar
(Mulheim an der Ruhr, DE) |
Assignee: |
Siemens Power Corporation
(Palmetto, FL)
|
Family
ID: |
25351997 |
Appl.
No.: |
07/868,607 |
Filed: |
April 14, 1992 |
Current U.S.
Class: |
60/39.182;
122/7R |
Current CPC
Class: |
F01K
23/10 (20130101); Y02E 20/16 (20130101) |
Current International
Class: |
F01K
23/10 (20060101); F02C 006/18 () |
Field of
Search: |
;60/39.181,39.182
;122/7R,7B |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Casaregola; Louis J.
Attorney, Agent or Firm: Zebrak; Ira Lee
Claims
What is claimed is:
1. A compound cycle power plant comprising a gas turbine having a
hot gas exit topped onto at least two steam turbine plants, having
heat exchange means between the gas turbine and the steam turbine
plants, the heat exchange means communicating with the hot gas exit
and arranged to pass hot gases exiting the gas turbine in heat
exchange with feedwater cycled through said steam turbine plants to
preheat said water or convert it to steam.
2. A compound cycle power plant comprising a gas turbine having a
hot gas exit topped onto at least two steam turbine plants, having
heat exchange means between the gas turbine and the steam turbine
plants, the heat exchange means communicating with the hot gas exit
and arranged to pass hot gases exiting the gas turbine in heat
exchange with feed water cycled through said steam turbine plants
to preheat said water or convert it to steam, wherein the heat
exchange means comprises at least two separate heat exchangers,
each communicating with the hot gas exit and one of each separate
heat exchangers communicating with recycle water from one of each
respective steam turbine plants.
3. The power plant of claim 2 which further comprises control means
for controlling the flow of hot gases from the hot gas exit to the
heat exchange means.
4. The power plant of claim 3 wherein the control means comprise a
first means for controlling the total amount of hot gas flowing to
the heat exchange means and a second means for controlling the flow
of hot gases to each respective heat exchanger.
5. The power plant of claim 4 wherein the first and second control
means are louver dampers.
6. The power plant of claim 3 which further comprises extraction
means for extracting heat from the heat exchanger for heating
purposes external of said steam turbine.
7. The power plant of claim 2 wherein two steam turbines are
connected to the gas turbine.
8. The power plant of claim 2 wherein the heat exchangers are
connected so as to only heat feedwater for the steam turbine
plants.
9. The power plant of claim 6 wherein one heat exchanger is
connected so as to heat feedwater for one steam turbine plant and
the other heat exchanger is connected so as to supplying heat to
the heat extraction means.
10. The power plant of claim 4 wherein the heat exchangers is
connected so as to provide hot reheat steam to the steam turbine
plants.
11. A compound cycle power plant comprising a gas turbine having a
hot gas exit topped onto at least two steam turbine plants, having
heat exchanger means between the gas turbine and the steam turbine
plants, the heat exchanger means comprising at least two separate
heat exchangers communicating with the hot gas exit and arranged to
pass hot gases exiting the gas turbine in heat exchange with
feedwater cycled through said steam turbine plants to preheat said
water or convert it to steam.
12. The power plant of claim 11 which further comprises control
means for controlling the flow of hot gases from the hot gas exit
to the heat exchange means.
13. The power plant of claim 12 wherein the control means comprise
a first means for controlling the total amount of hot gas flowing
to the heat exchange means and a second means for controlling the
flow of hot gases to each respective heat exchanger.
14. The power plant of claim 13 wherein the first and second
control means are louver dampers.
15. The power plant of claim 12 which further comprises extraction
means for extracting heat from the heat exchanger for heating
purposes external of said steam turbine.
16. The power plant of claim 11, wherein two steam turbines are
connected to the gas turbine.
17. The power plant of claim 11 wherein the heat exchangers are
connected so as to only heat feedwater for the steam turbine
plants.
18. The power plant of claim 16 wherein one heat exchanger is
connected so as to heat feedwater for one steam turbine plant and
the other heat exchanger is connected so as to supplying heat to
the heat extraction means.
19. The power plant of claim 13 wherein the heat exchangers are
connected so as to provide hot reheat steam to the steam turbine
plant.
Description
BACKGROUND OF THE INVENTION
This invention pertains to a topping arrangement of two or more
steam turbine plants with a gas turbine.
Combined cycle units with heat recovery steam generators (HRSG's)
as well as combined cycle plants with fully-fired steam generators
are known. As a result, a variety of combined cycle arrangements
based on simple plant power concepts have been developed. Thus,
combined cycle plants with fully fired steam generators have been
in operation as early as 1965. The gas turbines for these plant
concepts are equipped with a heat recovery steam generator or heat
exchanger utilizing the gas turbine exhaust energy to provide
additional main steam, reheat steam or feedwater heating for steam
plants, thereby increasing their output and overall power plant
efficiency. These power plant concepts designated as compound
cycles can be adopted for new facilities, but are especially
suitable for repowering or topping projects. Since only a minimum
of new steam plant equipment is needed, these plant concepts can
become attractive low cost projects. See Maghon, H., Bermann, D.,
Bruckner, H., Kriesten, W., and Termuehlen. "Combined Cycle Power
Plants for Load Cycling Duties" American Power Conference, Chicago,
Ill., April 1989; Kreutzer, A., Ganzer, W., and Termuehlen, H.,
"Gas and Coal-Fired Combined Cycle Plants" American Power
Conference, Chicago, Ill., April 1986; and Denizci, H., and Hamann,
B., "Design and Operation of Ambarli Combined Cycle Power Plant"
AEIC, Committee on Power Generation, Sep. 1991.
The largest plant built with a fully fired steam generator, also
known as a "hot wind box" design is in Germany. It features four
417 MW size units and one 700 MW unit providing a net plant output
of 2300 MW. The 770 MW unit features a coal-fired steam generator
with a desulfurization system. Since the reliability of the
previously installed gas turbines has been outstanding, it was
decided to provide only 60% forced draft (FD) fan capacity for the
operation of the steam plant without gas turbine, reducing the
output for this mode of operation from 656 MW to roughly 500 MW. In
the normal combined cycle operating mode, the gas turbine exhaust
is supplied as preheated air with about 16% oxygen to the steam
generator and its coal mills. A cooling air fan provides air to
control the temperature in the mills and for FD-fan operation a
primary air heater is installed. A partial-flow economizer provides
feedwater heating in parallel to the HP feedwater heaters of the
steam plant.
The fully-fired concept has also been applied for repowering a 590
MW power station. See Maghon, H., Schulenburg, T., Laakkonen, M.,
Froehlich, G., and Termuehlen H., "Full-Load Testing of the
Advanced V64.3 Gas Turbine" American Power Conference, Chicago,
Ill. April 1991. A V94.2 gas turbine has been installed and an
auxiliary FD-fan was provided to achieve maximum output in the
combined cycle operating mode with the two original larger FD-fans
being only used for back-up operation, without the gas turbine. A
flue gas bypass is provided for partial load operation of the steam
plant to reduce the hot air supply to the furnace. HP and LP
partial flow economizers are used in parallel to the HP and LP
feedwater heaters to further improve the overall plant efficiency
which was measured to be 46.6% or 7320 Btu/kWh at rated output.
Comparing this performance with the original reheat steam plant
efficiency of 40.7% (8,380 Btu/kWh) reveals a plant performance
improvement of 5.9 percent points or 1,060 Btu/kWh. Load cycling
operation between 100% and 45% plant output can be performed
without plant efficiency deterioration. The NO.sub.x emission of
the power plant was reduced to 30% of the original level from
400-500 ppm to 100-150 ppm at 3% oxygen content of the steam
generator's stack flue gas.
SUMMARY OF THE INVENTION
New alternatives are required for utilities in need for peaking
capacity or a combination of base load, mid-range load, and peaking
capacity. We have discovered that this can be provided in a
compound cycle plant comprising a topping arrangement of a gas
turbine onto two or more steam turbine plants. This system provides
high plant efficiency and excellent operating flexibility.
The apparatus of the invention comprises a gas turbine topped onto
two or more steam turbine plants having therebetween a heat
recovery heat exchanger comprising two side-by-side, but separate,
ducts. Each of the ducts comprises heat exchange means for each of
the respective steam turbines.
In accordance with the invention, the hot exhaust gases from a gas
turbine are passed in heat exchange with heat exchange means for
each of a steam turbine. The invention further comprises damper
means for controlling the amount of hot exhaust gas passing into
the respective heat exchange means for each steam turbine. In
another embodiment of the invention, separate damper means for each
of said respective heat exchange means for varying the amount of
hot exhaust gas between each of said heat exchange means are
provided.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of a gas turbine topped steam turbine
arrangement in accordance with the present invention.
FIGS. 2(A) and 2(B) are side and top views of a heat exchanger in
accordance with the present invention.
FIG. 3 is a side view of a gas turbine-generator with a heat
exchanger for feed water preheating in accordance with the present
invention.
FIG. 4 is a schematic drawing similar to FIG. 1 of another
embodiment of the invention.
FIG. 5 is a schematic drawing similar to FIG. 1 of yet another
embodiment of the invention.
FIG. 6 is a heat energy temperature diagram for the embodiment
depicted in FIG. 5.
FIG. 7 is a schematic diagram similar to FIG. 1 of yet another
embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The following examples illustrate the invention:
EXAMPLE 1
Compound Cycle Topping with Two Steam Turbine Units with One Gas
Turbine
As a basis, two identical 300 MW size reheat steam units were
chosen to closely reflect an actual application. It was further
assumed that the steam turbines were designed for a larger reheat
steam flow, typically required for operation with the top heater
out of service.
A maximum output of 326.8 MW per unit is generated when operating
with the valves wide open and a 5% overpressure. The plant net
performance was evaluated to be:
TABLE 1 ______________________________________ Output Rated Maximum
______________________________________ Turbine-Generator 312.2 MW
326.8 MW Auxiliary Load 15 MW 26 MW Plant Output 297.2 MW 310.8 MW
Turbine-Generator 7731 Btu/kWh 7709 Btu/kWh Heat Rate Plant Net
Heat Rate 9904 Btu/kWh 9885 Btu/kWh
______________________________________ Turbine-Generator Heat Rate
based on Low Heat Value Plant Heat Rate based on High Heat
Value
For this application, it was found that the potential output
increase can best be achieved when applying one V84.3 gas turbine
to provide feedwater heating or reheat steam for both units in a
compound cycle mode the V84.3 gas turbine performance for base and
peak capacity is at 59.degree. F. ambient temperature the
following:
TABLE 2 ______________________________________ Load Base Peak
______________________________________ Net Output 135.5 MW 145.9 MW
Heat Rate Based on 9767 Btu/kWh 9668 Btu/kWh Low Heat Value Heat
Rate Based on 10,838 Btu/kWh 10,728 Btu/kWh High Heat Value
______________________________________
The original plant net heat rate of 9554 Btu/kWh has been improved
for both reheat steam units to 9048 Btu/kWh with only the one gas
turbine being installed. The rated output was increased by:
______________________________________ One Gas Turbine Output 135.5
MW Additional Output Increase of Steam Plants 2 .times. 27.1 MW
Total Output Gain 189.7 MW
______________________________________
The maximum plant output increased, when operating the gas turbine
at peak-load capacity with a heat rate level of 9008 Btu/kWh, as
follows:
______________________________________ One Gas Turbine Output 145.9
MW Additional Output Increase of Steam Plants 2 .times. 28.8 MW
Total Output Gain 203.5 MW
______________________________________
The maximum performance for adding one V84.3 gas turbine generator
as a simple cycle peaking unit to the system compared to building a
completely new combined cycle plant or a compound cycle arrangement
is shown in the following three equations.
______________________________________ Addition of a Simple Cycle
V84.3 Plant Steam Turbine Plant 2 .times. 310.8 MW 9885 Btu/kWh Gas
Turbine Plant 1 .times. 148 MW 10570 Btu/kWh Total Plant 769.8 MW
10017 Btu/kWh Addition of a Combined Cycle V84.3 Plant Steam
Turbine Plant 2 .times. 310.8 MW 9885 Btu/kWh Gas Turbine Plant 1
.times. 221 MW 7070 Btu/kWh Total Plant 842.6 MW 9147 Btu/kWh
Addition of a Compound Cycle V84.3 Plant Steam Turbine Plant 2
.times. 339.6 MW 8955 Btu/kWh Gas Turbine Plant 1 .times. 145.9 MW
10728 Btu/kWh Total Plant 825.1 MW 9269 Btu/kWh
______________________________________
The higher gas turbine output for the simple cycle application is
the result of smaller outlet pressure losses.
Summarizing these results leads to the following improvements of
performance for adding combined cycle or compound cycle units
versus adding a simple cycle gas turbine unit:
TABLE 3 ______________________________________ Total Plant Simple
Combined Compound Net Performance Cycle Cycle Cycle
______________________________________ Output MW 769.6 842.6 825.1
Output Increase MW Bawe 73.0 55.0 % Base 9.5 7.2 Heat Rate Btu/kWh
10017 9147 9269 Improvement Btu/kWh Base 870 748 % Base 8.7 7.5
______________________________________
The smaller increase in output and decrease in heat rate for the
compound cycle versus the combined cycle is mainly a result of
increased losses in the steam turbines' LP sections. By replacing
these parts of the existing steam turbines the output gain and heat
rate performance for the compound cycle can be further
improved.
For selecting an optimal repowering concept for this unique
compound cycle application, we investigated this and three more
cycle concepts.
EXAMPLE 2
Alternative I
Feedwater Heating for a Plurality of Extractions
The V84.3 gas turbine exhaust energy is matching the required heat
for the two HP heaters' feedwater heating capacity of both reheat
steam plants as well as partial feedwater heating in parallel to
the LP extractions. The feedwater inlet temperature of 180.degree.
F. for extraction 3 matches the stack gas temperature of
200.degree. F., which provides an optimal overall performance.
During steam plant partial load operation recirculation can be used
to keep the feedwater temperature at about 180.degree. F. With the
available exhaust energy from the gas turbine, a final feedwater
temperature of 491.degree. F. was achieved. The maximum plant net
was increased as follows:
TABLE 4 ______________________________________ Reheat Plant
Compound Cycle ______________________________________ Steam Plant
Output 2 .times. 326.8 MW 2 .times. 355.6 MW Gas Turbine Output 0
MW 1 .times. 145.9 MW Total Output 653.6 MW 857.1 MW Auxiliary
Power 2 .times. 16 MW 2 .times. 16 MW Plant Net Output 621.6 MW
825.1 MW Plant Net Output Increase 203.5 MW
______________________________________
The arrangement provides this capacity increase at an attractive
heat rate improvement of 616 Btu/kWh or 6.2%. This improvement over
the reheat steam plant performance is very attractive and is a
result of optimal utilization of the gas turbine exhaust energy at
a high energy level.
Full-load operation of the gas turbine and partial-load operation
of the steam plants might be limited by the final feedwater
temperature reaching a level at which boiling in the reheat steam
boilers' economizers could occur. For controlling such operational
modes, the heat exchanger is of a special design, shown in FIG. 1,
providing extremely high operating flexibility.
The dual-duct design of the heat recovery heat exchanger 12 permits
any mode of operation of the one gas turbine and two steam plants.
An auxiliary stack 14 is provided to operate the gas turbine 16
without the steam plants 18 and 20, since flue gas flows are
controlled by three multiple-louver dampers 22, 24, and 26. The two
dampers 24 and 26 upstream of the heat exchangers 28 and 30 for the
two steam plants 32 and 34 provide independent flow and, in turn
temperature control for each of these units. With this arrangement,
Operating the gas turbine and only one steam plant is possible.
This system also allows start-up of one unit with the other steam
plant already in full operation.
The detailed design of the heat exchanger 12 for the feedwater
preheating is illustrated in FIGS. 2 and 3. The gas turbine exhaust
enters at first the single gas turbine stack 14. From there, the
gas flow is equally split into channels leading to two independent
HP and LP feedwater heating tube bundles. This dual-channel
arrangement has been chosen to operate the two steam plants
independently.
This design concept has been developed to provide the highest
degrees of operating flexibility. The gas flows are controlled by
louver dampers. Two rows of louvers 34 and 36 are installed in the
gas turbine auxiliary stack to minimize leakage losses during
operation of the HP and LP feedwater heat exchangers. Pressurizing
the area between these two rows of louvers eliminates any leakage
losses. In front of each heat exchanger section is a single row
louver damper 24 and 26 installed to control independently, the gas
flow to the HP and LP feedwater heat exchangers. Operating
experience with louver dampers in Europe have been very positive.
Modulating the three dampers in the auxiliary stack and up stream
of the two heat exchanger sections in the proper sequence permits
controlling the operations of:
only the gas turbine
the gas turbine with one steam turbine unit
the gas turbine with two steam turbine units
transfer from and to any of these operation modes
start-up to all these operating conditions
The gas turbine stack for the V84.3 gas turbine shown in FIG. 2 is
18 feet and the two discharge stacks of the heat exchanger sections
are 13 feet in diameter. Operation with one steam plant requires
partial opening of the auxiliary stack to optimize maximal
feedwater heat supply to this unit and a minimum of pressure losses
at the gas turbine exhaust.
The arrangement of the heat exchanger with a V84.3 advanced gas
turbine is illustrated in FIG. 3. The heat exchanger with its over
all length of 111'7" is connected to the axial diffuser of the gas
turbine. The overall length of the V84.3 gas turbine with a 165 MVA
generator is 75'7". Adding the diffuser length of 37' results in a
total length of 224'2" for the entire gas turbine/heat exchanger
plant.
As illustrated the advanced V84.3 gas turbines' two combustion
chambers are horizontally arranged. The gas turbine air intake at
the generator side is extended to the air filters which are mounted
above the air-cooled generators. The turbine inclosure width is
approximately 64 feet which provides space for auxiliaries and
internal maintenance work. The V84.3 gas turbine features 2.times.6
hybrid burners to provide low NO.sub.x emission without steam or
water injections in a premix burning mode. For operation with fuel
oil or for power augmentation, steam or water injection is
provided.
EXAMPLE 3
Alternative II
Feedwater Heating for Extractions E.sub.7 Through E.sub.3
This cycle shown in FIG. 4, provides the following increase in
maximum plant out put at a heat rate improvement of 389 Btu/kWh or
3.9%:
TABLE 5 ______________________________________ Reheat Plant
Compound Cycle ______________________________________ Steam Plant
Output 2 .times. 326.8 MW 2 .times. 345.9 MW Gas Turbine Output 0
MW 1 .times. 145.9 MW Total Output 653.6 MW 837.7 MW Auxiliary
Power 2 .times. 16 MW 2 .times. 16 MW Plant Net Output 621.6 MW
805.7 MW Plant Net Output Increase 184.1 MW
______________________________________
This arrangement with a dual-duct heat exchanger is similar to
alternative I, but provides partial feedwater heating for seven
feedwater heaters indicated as Extractors E1 through E7. At maximum
power plant load, about 57% of the feedwater is preheated by the
gas turbine exhaust energy and the remaining 43% by the steam
extractions from the steam turbines through their feedwater
heaters. This arrangement is not as efficient since the gas turbine
discharge energy is not utilized at an elevated level and
109.degree. F. condensate is heated by 200.degree. F. flue gas. To
avoid corrosion in the heat exchangers' discharge section,
recirculation pumps 38 and 40 are installed for raising the
condensate inlet temperature. Partial-load operation of the steam
plants without increasing the final feedwater temperature can be
performed by simply increasing the feedwater flow through the
feedwater heat exchangers.
EXAMPLE 4
Alternative III
Hot Reheat Steam Supply
Operation without the top feedwater heater in service basically
results in an increase in reheat steam flow and pressure. Instead
of utilizing the gas turbine exhaust for feedwater heating, this
alternative generates hot reheat steam in the heat recovery steam
generator which is then admitted into the reheat steam systems of
the steam plants, as illustrated in FIG. 5. The high gas turbine
exhaust temperature provides steam with about 1000.degree. F. The
compound cycle maximum output increased at an improved heat rate
level of 623 Btu/kWh (6.3%) is:
TABLE 6 ______________________________________ Reheat Plant
Compound Cycle ______________________________________ Steam Plant
Output 2 .times. 326.8 MW 2 .times. 353.5 MW Gas Turbine Output 0
MW 1 .times. 145.9 MW Total Output 653.6 MW 852.9 MW Auxiliary
Power 2 .times. 16 MW 2 .times. 17 MW Plant Net Output 621.6 MW
818.9 MW Plant Net Output Increase 197.3 MW
______________________________________
The heat recovery steam generator receives condensate at
109.degree. F. through a booster pump. The economizer features a
recirculation pump to avoid corrosion at the flue gas discharge
section. FIG. 6 shows the heat recovery steam generator design
criteria. The recirculation provides a condensate inlet temperature
of about 180.degree. F. with a correspondent stack gas temperature
of 200.degree. F. Roughly 36% of the heat energy is utilized in the
two economizer sections. The single-pressure HRSG has a pinch point
of 10.degree. F. at a 500.degree. F. temperature level and a
temperature difference between produced steam and gas turbine
exhaust of about 60.degree. F. The diagram reveals that more steam
could be produced if more than 64% of the gas turbines energy would
be utilized. Therefore, a lower pressure level (reheat steam
pressure of the reheat steam plants) would result in a better
overall cycle performance.
The natural circulation section of the HRSG produces hot reheat
steam. This relative simple arrangement provides high operating
flexibility. The condensate and reheat steam flows from and to the
two steam plants are controlled to balance the water and steam
supply for the HRSG and the two reheat steam plants. This control
concept also permits independent operation of the two reheat steam
units. Since no auxiliary stack is provided, gas turbine operation
is only possible with at least one steam plant in operation or at
partial load with a reduced exhaust temperature. Steam bypass
systems are supplied to provide operating flexibility like gas
turbine start-up with the steam units operating at full load.
EXAMPLE 5
Alternative IV
Hot Reheat Steam Supply and Feedwater Heating for Multiple Heat
Extractions
This compound cycle arrangement of a HRSG with feedwater heating
provides a greater performance increase than alternative III with a
heat rate improvement of 655 Btu/kWh (6.6%) and the following
maximum output:
TABLE 4 ______________________________________ Reheat Plant
Compound Cycle ______________________________________ Steam Plant
Output 2 .times. 326.8 MW 2 .times. 354.8 MW Gas Turbine Output 0
MW 1 .times. 145.9 MW Total Output 653.6 MW 855.5 MW Auxiliary
Power 2 .times. 16 MW 2 .times. 17 MW Plant Net Output 621.6 MW
821.5 MW Plant Net Output Increase 199.9 MS
______________________________________
As illustrated in FIG. 7, feedwater preheating is provided for
extraction E.sub.5, E.sub.4 and E.sub.3 from the low temperature
section of the HRSG. The steam generation section of the HRSG
receives feedwater from the steam plants' boiler feedwater pumps.
The feedwater and reheat stem flows from and to the two steam
plants are controlled as in alternative III.
Besides best performance, the operating flexibility is compatible
to alternative III, permitting full-load gas turbine operation with
only one steam plant in operation. In cases where the gas turbine
to steam turbine output ratio is larger or the reheat pressure is
lower, this cycle would become even more efficient because of the
increased reheat steam produced at a high energy level.
EVALUATION OF ALTERNATIVES I THROUGH IV
All four alternatives are valid options to be utilized for compound
cycle repowering. The two feedwater heat exchanger options seem to
be easier to integrate into existing plants, since only feedwater
piping connects the gas turbine plant with the steam plants. This
pipes's pressure and heat losses are not a major concern. On the
other hand, the heat recovery steam generator options provide a
slightly better performance improvement. At first, it was thought
that the feedwater heat exchangers should be much less expensive
than the HRSG's, however, a price comparison considering the use of
auxiliary stacks and dampers for the feedwater heat exchanger
options raise their cost.
The feedwater heat exchange options provide greater operating
flexibility, which is especially important if load cycling and
two-shift operation is considered.
In regard to thermal efficiency alternative IV is the best
solution, whereas the largest output increase is achieved with
alternative I, as listed in Table 8. The larger output increase of
alternative I is a result of the increased reheat steam production
in the steam plants. The plant net outputs and net heat rates of
the various alternatives reveal differences which must be part of a
proper evaluation of topping options.
__________________________________________________________________________
Present Reheat Steam Plants Alternative I Alternative II
Alternative III Alternative
__________________________________________________________________________
IV Gas Turbine Output MW 145.9 1 .times. 145.9 1 .times. 145.9 1
.times. 145.9 Steam Plant Output MW 2 .times. 326.8 2 .times. 355.6
2 .times. 353.9 2 .times. 353.5 2 .times. 354.8 Total Output MW
653.6 857.1 837.7 852.9 855.5 Auxiliary Output MW 2 .times. 16 2
.times. 16 2 .times. 16 2 .times. 17 2 .times. 17 Plant Net Output
MW 621.6 826.1 805.7 818.9 821.5 Plant Net Heat Rate Btu/kWh 9885
9269 9496 9262 9230 Based on High Heat Value Increase in Plant Net
MW Base 203.5 184.1 199.3 201.1 Output % Base 32.7 29.6 32.1 32.4
Improvement in Plant Btu/kWh Base 616 389 623 655 Net Heat Rate %
Base 6.2 3.9 6.3 6.6
__________________________________________________________________________
The gas turbine is influenced by differences in outlet pressure
loss of the heat exchangers and HRSG's, however, all calculations
were performed with 12 inch H.sub.2 O because it was assumed that a
lower pressure drop in the heat exchangers might be compensated by
the use of the auxiliary stack and dampers for these
alternatives.
A 165 MVA air-cooled generator has been selected for all
alternatives. However, a larger air-cooled generators can be
provided for operating at a lower power factor, in case the maximum
steam turbine generator capacity is limited and additional reactive
power is needed. Presently, air-cooled generator frame sizes up to
260 MVA are available.
The steam turbine-generators were originally designed for the
following potential flows:
______________________________________ Main Steam Reheat Steam
Exhaust Steam ______________________________________ 2,100,000
lb/hr 2,100,000 lb/h 1,600,000 lb/h
______________________________________
The maximum steam turbine output in a compound cycle operating mode
of 355.6 MW required the following flows.
______________________________________ Main Steam Reheat Steam
Exhaust Steam ______________________________________ 2,100,000
lb/hr 2,100,000 lb/h 1,710,000 lb/h
______________________________________
The exhaust steam flow is the only one which has been increased
above its original value. But this increase of only 7% can
generally be handled if the steam turbines are not backend loaded
or the LP blading is designed for a low mass flow limit. In this
case, the LP turbine steam path can be replaced with an advanced
design to provide improved performance and a backend loading up to
18,000 lb/ft.sup.2 h. Such an example is illustrated in Table 9 as
alternative IA revealing an addition performance improvement of 16
MW over alternative I for both steam turbines.
TABLE 9
__________________________________________________________________________
Total Plant Net Simple Combined Cycle Compound Cycle Performance
Reheat Plant Cycle HRSG Alt I Alt IA
__________________________________________________________________________
Output MW 621.6 769.6 842.6 825.1 841.1 Output Increase MW Base 148
221 203.5 219.5 % Base 23.8 35.6 32.7 35.3 Heat Rate Btu/kWh 9885
10017 9147 9269 9093 Heat Rate Btu/kWh Base +132 738 616 792
Improvement % Base +1.3 7.5 6.2 8.0
__________________________________________________________________________
+ "Worse Heat Rae
Applying a combined cycle instead of a simple cycle unit reduces
the heat rate level of the entire plant by (1.3%+7.5%) 8.8%. This
result is based on a combined cycle plant heat rate of 7070 Btu/kWh
at HHV and 6370 Btu/kWh at LHV, which accounts for a 53.6% combined
cycle plant efficiency. The improvement with the compound cycle
alternative I is 7.5% and can be even raised to 9.3% by adding
advanced LP turbine replacements. A similar, however, somewhat
small improvement can be expected for the combined cycle option
when supplying additional LP turbine replacements.
Combined cycle plants with heat recovery steam generators and with
fully-fired steam generators as well as compound cycle plants are
depending on the circumstance valid alternative for repowering or
topping applications. When summarizing our findings for the
repowering of two 300 MW steam plants with one advanced gas
turbine, the comparison of performance versus the two present
reheat units are revealed in Table 8 based on the maximum total
power plant net output. Alternative I provides the largest output
in crease of 32.7% and alternative IV achieves the best heat rate
decrease of 6.6%. It should be noted that the evaluation of cycle
performance differences is based on a constant main steam flow. A
similar study based on constant thermal capacity of the steam
plant's steam generators would show a relatively better performance
for alternatives III and IV with HRSG's.
Adding a simple cycle gas turbine plant to an existing steam plant
increases the weighted heat rate when operating at full-load,
however, the total output is increased by the peaking capacity of
the gas turbine. Adding a combined cycle plant with a heat recovery
steam generator provides the best performance in heat rate and
output. Depending on the specific plant conditions, similar results
can be achieved with a fully-fired topping arrangement. Utilizing a
compound cycle arrangement provides a somewhat lower output and
worse heat rate compared to the combined cycle alternative.
However, output and heat rate performance of this compound cycle
plant can be improved by replacing the LP turbines. The utilization
of advanced LP turbine sections provides a heat rate and output
improvement in the range of 2 to 3%. With this addition, the
performance of the compound cycle plant shows up as a slightly
lower output, but better heat rate than the combined cycle with
heat recovery steam generator.
In regard to emission discharge the specific emission per generated
kWh is reduced to all alternatives because the gas turbine with its
hybrid burners provides a low NO.sub.x emission in the range of 25
ppm without steam or water injections. An even larger improvement
can be achieved with a fully-fired combined cycle plant since the
hot exhaust gas from the gas turbine is used for secondary
combustion in the reheat steam generators. At the Eemscentrale
plant this concept, in combination with an exchange of the burners
in the steam generators, lead to a reduction of the NO.sub.x
emission from 400-500 ppm before repowering to 100-150 ppm after
the gas turbine was topped onto the existing 600 MW steam
plant.
In cases where large relatively new existing steam plants reveal a
potential for uprating, the compound cycle application is an
attractive option. This eliminates the need for installing anew
steam turbine plant with its auxiliary systems like condenser and
cooling water supply as well as avoiding a major rebuild of the
existing steam generators.
Without modifying the steam turbine flow path, increasing the
capacity of existing steam turbines might be limited. When
considering replacement of steam turbine sections, the compound
cycle concept can generate maximum additional output at a very
attractive heat rate level. Since an existing steam plant is
utilized, the specific cost for such compound cycle topping can be
as low as the expense of building a simple cycle gas turbine
plant.
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